H&R produced their FOLDING model shotgun from circa 1926 to WW2. This model was made in LARGE FRAME and SMALL FRAME sizes. .410 bore was offered in both frame sizes. It appears from the serial numbers that I've seen, that more than 20,000 of these were made. There were two distinct variations - up to around 1930 they were equipped with an extractor (hull was removed by fingers) and the second variation from 1931 on was equipped with an ejector (hull was automatically extracted and sent flying). I have a large frame in 12 ga - 2nd Var. and a small frame in .410 - 1st Variation. They are actually pretty nicely made shotguns.
As to availability of forestocks - the forestock, obviously, will be specific to the size of the frame - all had a recess milled in that would allow the trigger guard to set in when the piece was folded. I've not seen any of these offered for sale during the many years I've been collecting H&R firearms and parts. HOWEVER, contact Numrich at:
www.e-gunparts.com and either e-mail or (preferably) call one of their sales techs and enquire whether they have what you need (of course you'll need to determin whether yours is a large or small frame, variation doesn't matter). Numrich bought out the remaining parts from H&R Arms Co. when they closed up shop in Jan 1986. Good luck.